xfs: get rid of indirections in the quotaops implementation

Currently we call from the nicely abstracted linux quotaops into a ugly
multiplexer just to split the calls out at the same boundary again.
Rewrite the quota ops handling to remove that obfucation.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <david@fromorbit.com>